Field of the Invention
The invention concerns a method for magnetic resonance imaging, and a magnetic resonance apparatus and a data storage medium for implementing such a method.
Description of the Prior Art
In a magnetic resonance apparatus, also referred to as magnetic resonance tomography system, the body to be examined of an examination person, in particular a patient, is usually exposed, with the use of a basic field magnet, to a relatively high basic magnetic field of 1.5 or 3 or 7 Tesla for instance. In addition, gradient pulses are activated with the use of a gradient coil arrangement. Radio-frequency pulses, for instance excitation pulses, are then emitted via a radio-frequency antenna using suitable antenna devices, with the result that nuclear spins of particular atoms that are resonantly excited by these radio-frequency pulses are tilted by a defined flip angle relative to the magnetic field lines of the basic magnetic field. As the nuclear spins relax, radio-frequency signals known as magnetic resonance signals are emitted, and are received and then further processed using suitable radio-frequency antennas. From the raw data acquired in this way, the desired image data can ultimately be reconstructed.
For a particular measurement, a particular magnetic resonance sequence, also called a pulse sequence, must be activated, which is composed of a sequence of radio-frequency pulses, for instance excitation pulses and refocusing pulses, as well as gradient pulses to be activated that are appropriately coordinated therewith, and proceed in various gradient axes along various spatial directions. Timed to coincide with this, readout windows are set that define the time periods in which the induced magnetic resonance signals are detected.
With the magnetic resonance imaging using a magnetic resonance scanner, the homogeneity of the basic magnetic field in an examination volume is of utmost importance. Excessive variances in the frequency distribution of the nuclear spins may occur even with small variances in the homogeneity, so that lower quality magnetic resonance image data are recorded.
Shim units are known in order to improve the homogeneity in the examination volume. After a magnetic resonance device is installed at its installation site, fields that are present in the surrounding area can restrict the homogeneity of the main magnetic field, in particular around an isocenter of the magnetic resonance scanner. Therefore during the installation and commissioning of a magnetic resonance apparatus, frequently in conjunction with measurements, the shim unit is set such that a homogeneity of the basic magnetic field that is as homogenous as possible is established. For this purpose, basic shim settings are calculated during the installation and commissioning of the magnetic resonance apparatus.
The examination object from which (whom) data are to be recorded represents a further inhomogeneity source. If a person to be examined is introduced into the magnetic resonance device for instance, the material of the person's body interferes with the homogeneity. In order to counteract this problem, it is known to use an adjustable shim unit. In particular, shim coils are known for this purpose that are controlled with different shim currents so as to generate different compensation magnetic fields in order to improve the homogeneity.
In order to shim these interferences of the object to be examined, it is conventional, firstly when the shim unit is controlled by the basic shim settings obtained during the installation and commissioning of the magnetic resonance device, to perform a measurement of the field distribution by operation of the magnetic resonance scanner itself, after the person to be examined has been introduced into the patient receiving zone of the magnetic resonance scanner. On the basis of the basic shim settings, updated shim settings are determined by a control computer by taking the measured field distribution into account. The shim unit is then controlled using the updated shim settings in order to achieve as optimal a homogeneity as possible.